It is a common practice in TAB (tape automated bonding) or flip chip to form protruding connection electrodes (i.e., bumps) of gold, copper, solder or nickel, or of multiple layers of such metals at predetermined portions (electrodes) of a surface of a semiconductor chip having interconnects formed therein so that the semiconductor chip can be electrically coupled via the bumps to substrate electrodes or TAB electrodes. There are various methods usable for forming the bumps, such as electroplating, vapor deposition, printing, and ball bumping. In recent years, electroplating, which can form fine bumps and can be performed in a relatively stable manner, has been widely used, as the number of I/O in a semiconductor chip increases and the electrode pitch becomes smaller.
The electroplating method can be classified roughly into a jet method (or a cup method) and a dip method. According to the jet method, a substrate, such as a semiconductor wafer, is held in a horizontal position with a surface, to be plated, facing downward, and a plating solution is jetted upward onto the surface to plate the substrate. According to the dip method, a substrate is held in a vertical position in a plating bath and a plating solution is injected upward into the plating bath, while the plating solution overflows the plating bath during plating. Electroplating using the dip method has advantages of a small footprint and good release of bubbles which could adversely affect a quality of plating. Moreover, the electroplating using the dip method can be easily applied to a variety of wafer sizes. Therefore, the dip method is considered suitable for bump plating in which plating is performed on relatively large-sized holes and which requires a considerably long plating time.
Japanese laid-open patent publication No. 11-315383 discloses a technology for generating a downward air flow in a clean room that houses a plating apparatus therein to increase the cleanliness in the clean room for thereby preventing particles from being attached to a surface, to be plated, of a substrate when the substrate is transported in the clean room. However, in the dip electroplating process, since a processing bath exists below the substrate that is being transported, the processing bath presents an obstacle to the formation of the downward air flow. Hence it is difficult to generate a uniform downward air flow. Therefore, particles that are suspended in the air in the clean room cannot fully be removed, and as a result the particles are liable to be attached to the surface of the substrate.
Furthermore, in the dip electroplating process, after the substrate has been dipped in the processing liquid in the processing bath and then raised from the processing bath, the substrate is transported in the horizontal direction while the substrate is kept in a vertical position. As a consequence, the particles suspended in the air in the clean room are likely to be attached to the surface of the substrate that has been plated.
A dip electroplating apparatus typically has a substrate holder for holding a substrate in a vertical position. This substrate holder has a scaling member that defines a hermetically closed space surrounding a peripheral portion of the substrate, with feeding electrodes disposed in this hermetically closed space. The substrate is held by the substrate holder in a state such that the surface, to be plated, of the substrate is exposed. The substrate and the substrate holder are immersed together in the plating solution, and the exposed surface of the substrate is plated in the processing bath.
A plating apparatus performs various processes, such as a pre-treating process and a rinsing process, as well as the plating process, on the substrate. During these various processes, the substrate, together with the substrate holder, is immersed in respective processing liquids. When the substrate is raised from the processing bath, the processing liquid remains on the substrate and the substrate holder. If the substrate holder is transported with the processing liquid remaining thereon, the processing liquid may drop from the substrate holder, thus possibly causing contamination. Moreover, if the substrate holder, with the processing liquid remaining thereon, is moved to a subsequent processing bath, the processing liquid remaining on the substrate holder is mixed into a different processing liquid in the subsequent processing bath, adversely affecting the processing of the substrate in the subsequent processing bath.
Attempts have been made in order to remove the processing liquid from the substrate and the substrate holder via gravity drop. For example, the substrate holder is lifted at a reduced speed from the processing bath, and kept above the processing bath after having been lifted therefrom. However, these approaches are liable to increase a takt time of the plating apparatus and as a result reduce a throughput.